Micromechanical tuning fork angular rate sensor

ABSTRACT

A micromechanical tuning fork gyroscope includes a suspended structure comprising at least first and second vibratable structures. Each vibratable structure is energizable to vibrate laterally, within a first plane, along an axis normal to the rotation sensitive axis. The lateral or inplane vibration of the first and second vibratable structures effects simultaneous vertical or rotational movement of at least a portion of the suspended structure upon the occurrence of angular rotation of the gyroscope about the rotation sensitive axis. Vertical or rotational movement of the suspended structure is sensed, and a voltage proportional to the movement is generated, for providing an indication of angular rate of rotation detected by the gyroscope.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.08/204,187 filed Mar. 4, 1994, abandoned which is a national phaseapplication of a PCT application PCT/US92/07685 filed Sep. 11, 1992,which is a continuation-in-part of U.S. patent application Ser. No.07/757,706 filed Sep. 11, 1991 (now abandoned).

FIELD OF THE INVENTION

This invention relates to angular rate sensors such as gyroscopes, andmore particularly, to a micromachined tuning fork angular rate sensorfabricated from a unitary silicon substrate.

BACKGROUND OF THE INVENTION

Angular rate sensors such as gyroscopes serve as one of the majorsensors in inertial navigation and guidance systems. Traditionally,gyroscopes have been implemented as large mechanical devices such asrotating wheel gyroscopes or other large metallic devices withdimensions of inches. These types of devices have several drawbacksincluding size restriction, reliability concerns due to the large numberof mechanical parts, and high costs associated with precise machinetolerances and tuning.

The use of tuning forks for gyroscopes has been attempted but has beenmet with limited success. These devices are costly due in part to theeffort required to tune the motor resonant frequency to the outputresonant frequency and also in part due to the large size of suchdevices. Thus, large scale production of these types of devices have notproven commercially viable.

In addition, prior art tuning fork gyroscopes have not been able toseparate or isolate the drive axis from the sense axis, leading tocausing more difficult and complicated manufacturing techniques.

Although a number of attempts have been made to produce solid state,micromachined tuning fork gyroscopes, the prior art devices are made ofquartz and still generally of intermediate size. In addition, precisionfabrication and assembly techniques as well as assembly costs havelimited the success of such devices.

SUMMARY OF THE INVENTION

This invention features a monolithic, double tined, close-end,micromechanical tuning fork gyroscope, for detecting angular rotationabout at least one rotation sensitive (sense) axis. Such a gyroscope isfabricated from a unitary silicon substrate in which has beenselectively etched a pit over which is suspended a non-etched siliconstructure.

The non-etched silicon structure is disposed within a first plane andincludes at least first and second vibratable structures. The first andsecond vibratable structures are disposed generally adjacent andparallel to one another. Each of the first and second vibratablestructures includes a mass integral with an associated vibratablestructure.

In one embodiment, the non-etched silicon structure is suspended overthe etched pit by first and second flexible elements which are disposedgenerally co-linear with the rotation sensitive axis of the gyroscope,for allowing the non-etched silicon structure to rotate about therotation sensitive axis.

The non-etched silicon structure may be divided into at least twoelectrically isolated yet structurally coupled segments. Each segmentincludes one vibratable structure. The vibratable structures areenergizable, in response to an applied voltage, to vibrate in a resonantor non-resonant mode parallel to a motor or drive axis orientedorthogonal to the sense axis.

Drive means energize the first and second vibratable structures tovibrate laterally, parallel to an axis normal to the rotation sensitiveaxis. Lateral vibration of the vibratable structures effects vertical orrotational movement of at least a portion of the non-etched siliconstructure about the rotation sensitive axis upon the occurrence ofangular rotation of the gyroscope about the rotation sensitive axis.Means for sensing rotation of the non-etched silicon structure areprovided, for sensing vertical or rotational movement of the non-etchedsilicon structure, and for providing a voltage proportional to therotational movement occurring in the non-etched silicon structure, forgenerating an indication of the angular rate of rotation detected by thetuning fork gyroscope.

DESCRIPTION OF THE DRAWINGS

These, and other features and advantages of the present invention willbe better understood by reading the following detailed description takentogether with the drawings, wherein:

FIG. 1 is a top view of a schematic representation of a double-tined,pivoting, closed-end tuning fork gyroscope according to one embodimentof the present invention;

FIG. 2 is a cross-sectional view of the gyroscope of FIG. 1 taken alonglines 2--2;

FIG. 3 is an enlarged view of the electrical isolation gap anddielectric lap joint according to the present invention;

FIGS. 4A-4C are top views of alternative designs for the isolation gapof the gyroscope according to the present invention;

FIG. 5 is a top view of a schematic representation of an open-endedtuning fork according to another embodiment the present invention;

FIG. 6 is a top view of a schematic representation of a non-pivotingtuning fork gyroscope according to yet another embodiment of the presentinvention;

FIG. 7 is a cross-sectional view of a beam member of the gyroscope ofFIG. 6 taken along lines 7--7;

FIGS. 8A and 8B are schematic representations of several alternativemethods of fabricating vibratable structures or tines for the gyroscopeof the present invention;

FIGS. 9A and 9B are schematic representations of a three axis instrumentmeasurement unit including a plurality of micromechanical gyroscopes ofthe present invention along with other measurement devices;

FIG. 10 is a schematic representation of a pivoting, closed end, tuningfork gyroscope according to one embodiment of the present inventionadapted for electromagnetic drive and electrostatic sense electronics;

FIG. 11 is a more detailed schematic representation of theelectromagnetic drive and electrostatic sense electronics of FIG. 10;

FIG. 12 is a representation of a two-piece, pivoting, closed end, tuningfork according to another embodiment of the present inventionincorporating electrostatic drive and sense electronics;

FIG. 13 is a more detailed schematic of the electrostatic drive andsense electronics of FIG. 12;

FIG. 14 is a schematic representation of a non-pivoting, closed endtuning fork according to yet another embodiment of the present inventionincluding electromagnetic drive and electrostatic sense electronics;

FIG. 15 is a schematic of electromagnetic drive and electrostatic senseelectronics including a closed-loop rebalancing circuit according to yetanother embodiment of the present invention;

FIG. 16 is a top view of a schematic representation of an alternative,pivoting, tuning fork gyroscope according to the present inventionadapted for electrostatic drive and sense electronics;

FIG. 17 is a schematic of the electrostatic drive and sense electronicsfor FIG. 16;

FIG. 18 is a more detailed schematic diagram of one embodiment of avibratable structure for the tuning fork gyroscope of the presentinvention;

FIG. 19 is an illustration of an alternative embodiment of the gapbetween the vibratable structure and the tilt plate of a tuning forkgyroscope according to the present invention;

FIG. 20 is a top view of a schematic representation of yet anotherembodiment of a pivoting, tuning fork gyroscope according to the presentinvention adapted for electromagnetic drive and electrostatic senseelectronics;

FIG. 21 is a schematic of the electromagnetic drive and electrostaticsense electronics for the gyroscope of FIG. 20;

FIG. 22 is a cross-sectional view of a surface micromachined,micromechanical tuning fork angular rate sensor according to anotherembodiment of the present invention;

FIG. 23 is an expanded cross-sectional view of a layered or sandwichedconstruction of a micromechanical tuning fork angular rate sensoraccording to yet another embodiment of the present invention; and

FIG. 24 is a cross-sectional view useful in describing themicromechanical tuning fork angular rate sensor of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

There is shown in FIG. 1 a monolithic, double-tined, pivoting,closed-end tuning fork gyroscope according to one embodiment of thepresent invention. The tuning fork gyroscope is micromachined from aunitary silicon substrate 10 employing the techniques of selective oxideremoval, boron diffusions into the substrate through the removed oxideregions, and anisotropic etching. Such methods are known in the art andare further described herein and are also disclosed in Applicant's U.S.Pat. No. 5,195,371 entitled "Method and Apparatus for Semi-ConductorChip Transducer" which is incorporated herein by reference.

The etch stop diffusions and subsequent selective anisotropic etchingcreate etch pit 12 having sloped sidewalls 16. Suspended over etched pit12 is a non-etched silicon structure 14.

The non-etched silicon structure 14 includes at least first and secondsegments 18 and 20. First and second segments 18 and 20 are electricallyisolated from one another by means of isolation gaps 22 and 24. Thesegments are structurally coupled by means of dielectric lap joints 26and 28 (shown in dashed lines) which bridge the isolation gaps therebystructurally coupling the segments together while retaining electricalisolation.

The non-etched silicon structure 14 further includes first and secondvibratable structures 38 and 40. The first and second vibratablestructures 38 and 40 form the two "tines" or "forks" of the tuning forkgyroscope according to the present invention.

The first vibratable structure 38 is coupled to first segment 18 of thenon-etched silicon structure, while the second vibratable structure 40is coupled to the second segment 20 of the non-etched silicon structure.Thus, the two vibratable structures are electrically isolated from oneanother. The first and second vibratable structures 38 and 40 areenergizable to vibrate in a resonant mode, and may be drivenelectrostatically or electromagnetically against each other parallel toan axis indicated by arrow 42 which is normal to the rotation sensitiveor input axis of the gyroscope indicated by arrow 44. Excitation ordrive signals, described in greater detail below, drive the twovibratable structures parallel to axis 42 by applying a sinusoidalvoltage of opposite polarity to each of the structures.

For improved performance, however, it may be desirable to add additionalmass at an appropriate location on the vibratable structures. Thislocation is usually at the center of a closed-end tuning fork and at theend of an open-ended tuning fork. Typically, the mass should besymmetrical about the plane of motion to avoid unwanted torques on thegyroscope structure.

The oscillating or vibrating inertia of first and second vibratablestructures 38 and 40 may thus be enhanced by adding masses 48 and 50 tothe first and second vibratable structures respectively. Masses 48 and50 are formed from silicon along with the non-etched silicon structure14 by selective doping and subsequent anisotropic etching. Sensitivityof the gyroscope may be further enhanced by adding additional weight tomasses 48 and 50 by plating gold, lead or a similar metal of highdensity. These metals and other high density metals may also bedeposited by chemical vapor deposition proximate or within tub regions52 and 54 formed in masses 48 and 50. By properly selecting the heightof the plating and the area to be plated above the plane of thenon-etched silicon structure, the center of gravity of the vibratablestructures may be positioned within the plane of the flexures, thusbalancing the masses about the plane of motion.

An important feature of the monolithic, micromechanical tuning forkgyroscope of the present invention is the ease of fabricating such adevice from a single unitary silicon substrate. Utilizing conventionalwell known photolithography techniques applied to the surface plane ofthe wafer, great precision in locating the mass may be obtained.

In one nominal design, the non-etched silicon structure 14 measuresapproximately 800 microns in length by 500 microns in width, whilemasses 48 and 50 are approximately 80 microns wide by 50 microns long by15 microns high, and weigh approximately 1.2×10-9 kg.

The first and second vibratable structures or forks are provided withenhanced flexibility in the direction parallel to axis 42 by choosingtheir geometry to include a high height-to-width ratio. In one design,each vibratable structure is approximately 4 microns wide by 15 micronshigh by 700 microns long. A gap of approximately 4 microns separates thetwo vibratable structures at their closest point.

The non-etched silicon structure 14 is suspended over etched pit 12 bymeans of flexible elements or pivots 56 and 58. One end of each of theflexible elements 56 and 58 is attached to diametrically opposing sidesof the non-etched silicon structure 14. The flexible elements aredisposed generally co-linear with rotation sensitive or input axis 44.The second end of each of the flexible elements remains integral withsilicon substrate 10. The flexible elements are typically doped with aP-type dopant for etch resistance, and are therefore electricallyisolated from the N-type silicon substrate 10 by means of a PN junctionwhich is formed between the P-doped flexible elements and the N typesilicon substrate.

The boron diffusion used to define the non-etched silicon structure andthe subsequent process of etching the surrounding silicon causesshrinking in the silicon lattice that creates a high tensile force inthe flexible elements. Accordingly, tension relief beams 60 and 62disposed proximate the first end of flexible elements 56 and 58 areformed by openings 64 and 66 in the non-etched silicon structure.Tension relief beams 60 and 62 serve to relieve the stress caused by thehigh tensile forces by allowing the tension relief beams to deflect orbow slightly, thereby reducing the torsional stiffness of flexibleelements 56 and 58 and thus increasing the rotational sensitivity ofnon-etched silicon structure 14.

Tension relief beams and various embodiments thereof and a method fortrimming the resonant frequency of a structure employing tension reliefbeams are described in greater detail in co-pending U.S. patentapplication Ser. No. 470,938 entitled "MICROMECHANICAL DEVICE WITH ATRIMMABLE RESONANT FREQUENCY STRUCTURE AND METHOD OF TRIMMING SAME"assigned to the assignee of the present invention, and which isincorporated herein by reference. Additionally, although first andsecond tension relief beams 60 and 62 are shown in this embodiment, onlyone tension relief beam is required to relieve the stress created by thehigh tensile forces in the flexures.

During operation of the gyroscope of the present invention, any angularrotation of the gyroscope about rotation sensitive axis 44 imparts anout-of-plane motion perpendicular to the laterally vibrating structures38 and 40. This motion produces a torsional motion in non-etched siliconstructure or plate 14, effecting an oscillatory rotational movement ofthe non-etched silicon structure about the rotation sensitive axis 44 asshown by arrow 67. The outer regions 68 and 69 of the non-etched siliconstructure 14 serve as "sense regions" of the gyroscope for detectingrotational motion resulting from an input angular rate. These "senseregions" may be thinner than the rest of the structure or alternatively,may include a "waffle" type construction, to minimize unwanted inertiaeffects.

In one embodiment, rotational sensing of the non-etched siliconstructure 14 about the rotation sensitive axis 44 is accomplishedutilizing bridge sense electrodes 70 and 72, although sensing andclosed-loop rebalancing of the non-etched silicon structure 14 utilizingburied sense electrodes 74 and 76 is also contemplated by the presentinvention. Bridge electrodes 70 and 72 extend from silicon substrate 10,over etched pit 12 and continue extending over the non-etched siliconstructure in the area of outer regions 68 and 69.

Typically, the bridge electrodes 70 and 72 are spaced approximately 2 to10 microns above the non-etched silicon structure. The bridge electrodesare coupled to electrostatic sense electronics 78 which measure theamount of rotation of the non-etched silicon structure by sensing thedifferential capacitance between the sense electrodes and the adjacentnon-etched silicon structure. Sense electronics 78, described in greaterdetail below, as well as drive and rebalance electronics may be formedon the silicon substrate adjacent to the tuning fork gyroscope of thepresent invention, or alternatively, may be located remotely from thegyroscope.

The torsional rotation imparted onto the pivoting non-etched siliconstructure 14 upon the occurrence of an angular rate input about rotationsensitive axis 44 is shown in greater detail in FIG. 2 wherein is shownflexible element 56 co-linear with rotation sensitive axis 44.Non-etched silicon structure 14 is torsionally displaced away frombridge sense electrode 70 and nearer to bridge sense electrode 72 uponthe occurrence of angular rotation about rotation sensitive axis 44.Before forming the bridge electrodes, the planar surface of the siliconsubstrate 10 is thermally oxidized to form a dielectric layer 80 whichelectrically isolates the bridge electrodes from the substrate. One ormore P+ doped buried electrodes such as electrodes 74 and 76 may beprovided.

A portion of the double tined, closed-end micromechanical tuning forkgyroscope of the present invention is shown in greater detail in thecross-section of FIG. 3 wherein is more clearly illustrated electricalisolation gap 22 which serves to electrically isolate first non-etchedsilicon structure segment 18 and the attached first vibratable structure38, from the second non-etched silicon structure segment 20 and theattached second vibratable structure 40. Electrical isolation gap 22 isformed by an intentional gap in the boron diffusion pattern utilized toform the non-etched silicon structure.

After the boron diffusion step but prior to anisotropic etch, dielectriclap joint 26 is formed over the electrical isolation slot by firstdepositing a layer of silicon nitride 80 of approximately 0.2 to 2.0microns in thickness. Subsequently, a layer of silicon dioxide 82 offrom 0.5 to 5 microns in thickness may be applied over the siliconnitride layer to stiffen the dielectric lap joint. By minimizing thewidth of the electrical isolation gap 22 to approximately 5 microns orless in the underlying silicon structure, and by applying dielectric lapjoint 26, a rigid and structurally coupled but electrically isolatednon-etched silicon structure results. The subsequent process ofanisotropic etching undercuts the silicon structure forming theelectrical isolation gap 22.

As shown in FIG. 1, isolation gap 22 is a diagonal gap in the non etchedsilicon structure 14. The tuning fork gyroscope of the present inventionalso contemplates other embodiments of the gap including a straight gap22a shown in FIG. 4A, and the undulating and non-straight isolation gaps22b and 22c shown in FIGS. 4B and 4C, which provide greater resistanceto bending than does gap 22a.

An additional embodiment of a tuning fork gyroscope according to thepresent invention is shown in FIG. 5 wherein is illustrated anopen-ended tuning fork fabricated from a unitary silicon substrate. Thegyroscope 100 includes a non-etched silicon structure 102 suspended overan etched pit 104 anisotropically etched from the silicon wafer.

Non-etched silicon structure 102 includes first and second vibratablestructures or forks 106 and 108 coupled to a rigid connecting segment110. A second pair of vibratable structures or forks 112 and 114 arealso coupled to rigid connecting segment 110.

The first and second vibratable structures 106,108 and rigid centralconnecting segment 110 are formed by relatively deep Boron diffusionsyielding structures with a height-to-width ratio of greater than 1. Thishigh height-to-width ratio gives the first and second vibratablestructures 106,108 a preferred oscillation in a direction of arrow 116.In addition, performance of the gyroscope according to this embodimentmay be enhanced by providing masses 118 and 120 at the ends or tips ofthe vibratable structures 106,108 respectively.

Applying a drive input voltage to protruding cantilevered electrodes 122and 124 will cause the first and second vibratable structures 106,108 tovibrate in a direction of arrow 116. Electrodes 122 and 124 are formedby the same diffusion that is used to form structure 110. In response toangular rotation about input axis 126, an oscillatory torque will beapplied to the rigid central segment 110. This oscillatory torque willcause the second pair of vibratable structures 112,114, having a lowheight-to-width ratio, to resonate in a vertical direction,perpendicular to the plane of the device and perpendicular to thelateral oscillation in a direction of arrow 116 of the first and secondvibratable structures 106,108.

The amplitude of the oscillation in the second set of vibratablestructures 112,114 is proportional to the angular rate, and is sensed bysense electronics 128 by applying a high frequency out-of-phase signalfrom sense input 134 to vibratable structures 112,114 through buriedelectrodes 130,132.

The structure may also include at least one opening 136 which formstension relief beam 138 for relieving stress form due to the Borondiffusions. Further, since the semiconductor material is N type and thenon-etched silicon structure 102 and bridge electrodes 122,124 are Ptype, all structures are electrically isolated from the body of thesilicon wafer by the PN junction formed between the P type structure andthe N type silicon wafer.

An additional embodiment of a positioning monolithic, micromechanicaltuning fork gyroscope according to the present invention is shown inFIG. 6 and includes, among other elements, first and second vibratablestructures 150 and 152. Each vibratable structure includes a mass suchas mass 154 coupled to first and second beams 156 and 158. The mass andattached beams vibrate as an entity with the beams operating as springs,which avoids the problem of the mass not being perfectly centered aboutthe vibrating beam and thereby causing unwanted torques. In the case ofa non-pivoting structure, the mass is disposed about the center of thetine. For readout purposes, bridge electrodes (not shown) may beprovided to sense the motion of the mass by measuring capacitancebetween the electrodes and sense regions 160,162.

Lateral and vertical vibration of the vibratable structures 150 and 152are facilitated by the geometry of the beams, such as beam 164 which isshown in cross-section in FIG. 7. Beam 164 includes first and secondsegments 166,168 which have a high height-to-width aspect ratio which istypically greater than 1. The high aspect ratio of these sections of thebeam provide added flexibility in the lateral direction, within theplane of the device as indicated generally by arrow 174, while alsoproviding increased resistance to vertical motion, perpendicular to theplane. In contrast, segments 170 and 172 have a much lowerheight-to-width ratio, on the order of 0.5 or less, which facilitatesmovement in the vertical direction indicated generally by arrow 176. Thelower height-to-width ratio segments of the beam therefore resistsmovement in the lateral direction, parallel to the plane.

The use of deep boron diffusion to form the flexible tines or vibratablestructures 38 and 40 of the tuning fork requires a significant gap, onthe order of three microns, between the tines to insure that they do notdiffuse together. Additionally, the boron also diffuses outwardly awayfrom the original photolithographic line. This tends to limit or reducethe desired height-to-width ratio which allows the tines to be flexiblein the lateral direction but stiff in the vertical direction.

The tuning fork gyroscope according to the present invention requiresthat the vibratable structures have a preferred mode of resonanceparallel to the plane of the device. Accordingly, the height of thetines or beams which comprise a portion of the vibratable structures ispreferred to be greater than their width, thus, a structure with a highaspect ratio. Vibratable structures comprised of high aspect ratio beamsmay be fabricated by selective boron diffusions through a slot in thesilicon and the silicon oxide, selective epitaxial back filling andcapping with another boron diffusion and subsequent anisotropic etchingto define a beam with an aspect ratio of up to 10-to-1. Such a method isdescribed in Applicant's U.S. Pat. No. 5,408,119 entitled "MONOLITHICMICROMECHANICAL VIBRATING STRING ACCELEROMETER WITH TRIMMABLE RESONANTFREQUENCY" which is incorporated herein by reference.

Several additional methods may be utilized to fabricate tines or beamsof varying aspect ratios. For example, two relatively shallow P+ Borondiffusions 180 and 182, FIG. 8A, may be provided in the N substrate 183.Anisotropic etching using a standard EDP process will result in twotines or beams defined generally by the P+ Boron doped areas 180,182,with an aspect ratio on the order of 0.5. Although not a very highaspect ratio, such beams might be useful as the main vibratablestructures of a tuning fork according to the present invention, or as alower aspect ratio portion of a beam such as beam 164, FIG. 7.

An additional method of fabricating tines is illustrated in FIG. 8B andincludes applying a very deep Boron diffusion in the area defined byline 184. Subsequently, a plasma etching process is used to cut throughthe etch resistant Boron doped material in areas 186. A subsequent EDPetch can then be used to undercut the structure leaving tines 188,190with a high aspect ratio. Aspect ratios of 1 to 4 are generally possiblewith this process.

One feature of the micromechanical, monolithic tuning fork gyroscope ofthe present invention is the ability to provide drive and senseelectronics on the semiconductor substrate itself that includes thegyroscope. Thus, one integral package includes both the gyroscope andthe necessary drive and sense electronics. In addition, two gyroscopes,one disposed orthogonal to the other, may be located on one substrate,thus allowing for two input axes on one substrate. Further, by providinga second tuning fork gyroscope substrate according to the presentinvention disposed in a plane perpendicular to the first substrate, athree axis tuning fork gyroscope system with one redundant axis may beconstructed.

For example, a first silicon substrate 300, FIG. 9A, mounted to mountingblock 302 or other device includes first and second gyroscopes withmutually orthogonal in plane input axes as shown by arrows 304 and 306.By providing a second semiconductor substrate 308 mounted in a planeperpendicular to the mounting plane of first substrate 300, a thirdgyroscope with an in plane input axis parallel to arrow 310 provides acomplete three axis gyroscope system with one input axis 312, parallelto axis 304, which is redundant or unused. Thus, only two mountingsurfaces are required to provide a three axis gyroscope system.

In a further embodiment, silicon substrate 320, FIG. 9B may include afirst gyroscope 322 which has an out-of-plane input axis perpendicularto the planar surface of substrate 320 parallel to arrow 324. Such agyroscope is described in U.S. Pat. No. 4,598,585. In addition,gyroscopes 326 and 328 according to the present invention each with anin-plane input axis, provide a 3-axis gyroscope system on one,semiconductor substrate.

The substrate also includes first and second in-plane sensingaccelerometers 330,332 such as a vibrating string accelerometerdisclosed in U.S. Pat. No. 5,408,119 entitled: MONOLITHICMICROMECHANICAL VIBRATING STRING ACCELEROMETER WITH TRIMMABLE RESONANTFREQUENCY which is incorporated herein by reference. Also included is athird accelerometer 334 having an out-of-plane input axis. Such anaccelerometer is disclosed in U.S. Pat. No. 5,126,812, entitled:ADVANCED MICROMECHANICAL ACCELEROMETER which is also incorporated hereinby reference. Thus, by further providing the three orthogonally sensingaccelerometers 330,334 a complete, three axis instrument measurementunit including all drive and sense electronics is provided on onesubstrate 300.

Various drive and sense electronics as well as a number of modes ofoperation may be utilized with various embodiments of the tuning forkgyroscope according to the present invention. For example, the tuningforks may be driven and sensed electromagnetically or electrostatically;in either open or closed loop; and the gyroscope may be designed to beself-oscillatory wherein an electronic loop may be used to maintain thetuning fork gyroscope at its natural or resonant frequency over a widetemperature range. In addition, consideration must be given to whetherthe tuning fork gyroscope will be fabricated on a pivoting platesupported by flexures or whether a non-pivoting device will be utilized.

The choice between pivoting and non-pivoting tuning fork gyroscopeconfigurations depends on a number of factors. For example, an advantageof the pivoting structure is that the lateral vibration of the tuningforks is separated from vertical movement on the plate which resultsfrom rotation of the gyroscope about the input axis. Since thegyroscopic forces on the tuning forks are transmitted to the pivotingstructure or plate, it is therefore not necessary to detect the bendingor vertical deflection of the tines themselves. Accordingly, the tinesdo not have to do double duty, vibrating laterally and deflectingvertically in a manner that lends itself to readout. Instead, verticalmovement is turned into an angular deflection of the plate or non-etchedsilicon structure which is therefore only dependent upon the flexureswhich connect the plate to the silicon frame. The flexures can thereforebe properly and advantageously designed independently of the tuningforks.

Additionally, in a pivoting gyroscope, the plate area can be madearbitrarily larger than the tuning forks, since it does not play a partin the gyroscopic action. This means that there will be a much largerreadout sensitivity for the pivoting gyroscope then for the non-pivotingversion.

Several of the many possible combinations of gyroscope electronics andconstruction configurations will be detailed below for illustrativepurposes, although many more combinations are possible and contemplatedby the present invention. In the first example, pivoting gyroscope 200,FIG. 10, is illustrated with electromagnetic drive electronics 202 andelectrostatic sense electronics 204. Drive electronics 202 apply an ACvoltage to flexure 206 which is coupled to rotatable plate 208. Plate208 includes a high conductive layer or wire 210 which begins at one endof flexure 206, but not on the flexure itself. Wire or conductive layer210 splits symmetrically and passes on or through the first and secondvibratable structures or forks 212,214, then recombines into a singlewire terminating at the second flexure 216. Output 218 from the secondflexure 216 is then coupled to electrostatic sense electronics 204 whichprovides an output voltage signal 220 whose voltage is proportional tothe input rate. This signal is subsequently processed utilizing wellknown signal processing techniques to provide the gyroscope outputsignal.

Tuning fork gyroscope 200 is also provided with electrodes222a,222b,224a,224b, and 226a,226b. Electrodes may be disposed eitherbelow plate 208 (buried electrodes) or above plate 208 (bridgeelectrodes), and are arranged symmetrically along both sides of theplate. The electrodes are used for torquing the plate in the case of aclosed-loop mode, and for sensing the angular displacement of the plate.Dielectric isolation gaps 228 and 230 are provided to eliminate anypotential voltage gradient on plate 208.

A more detailed schematic representation of electromagnetic drive andelectrostatic sense electronics 202 and 204 is shown in FIG. 11 whereinan AC drive voltage E_(f) from voltage source 232 is applied throughresistor 234 to the tuning forks 212,214 and plate 208, all representedby signal path 236. Drive voltage E_(f), typically 1-10 volts at 1-10Khz causes tuning forks 212,214 to vibrate laterally. An excitationvoltage source 238 provides an excitation voltage E_(x), typically 1volt at 0.1 to 1 Mhz, which serves as a reference signal to indicaterotational movement of plate 208 caused by an input rate about the senseaxis. The inverted and non-inverted signals are applied to oppositesides of the tuning fork structure. If it is desired to operate thedevice in a closed-loop mode, two torque and bias voltages E_(t1), V₁and E_(t2), V₂, 240-246 respectively are provided to rebalance therotational movement of the plate in phase and in quadrature with thevibration of the forks as will be explained in greater detail below.

The operation of the circuit is dependent upon the fact that output 218from the flexures 216 is driven to virtual ground by the feedbackcircuit of operational amplifier 248. Angular rate causes the deflectionof the tiltplate 208. The tilt is caused by the torque exerted upon thetiltplate from gyroscopic forces when there is an angular rate about theinput axis. The tilt of the plate is measured by the capacitor sensorsystem made up of electrodes 222a, 222b, 224a, 224b, 226a and 226b asshown. The electrodes may also be used to torque the plate externallysuch as when a rebalance loop is used as described below.

The angle of rotation of the tiltplate is sensed by exciting one set ofelectrodes with Ex, a high frequency AC signal. By demodulating thesignal using demodulator 250, with respect to the vibration frequency wxwx of the tuning forks, an envelope or modulating signal eo2 isrecovered and provides a voltage proportional to the tilt angle. Thisvoltage is proportional to the input rate when the gyroscope is operatedin the open loop mode or, when the gyroscope is operated in the closedloop mode, the voltage is used to provide a signal to rebalance thetiltplate by means of the above mentioned electrodes.

The torquing voltage is proportional to the applied torque on thetiltplate by virtue of a conditioning system which makes use of therelations in the following equations to convert the square lawforce-voltage relationship for a capacitor into a linear voltage-torquerelationship.

    e.sub.222a =-E.sub.x ; e.sub.222b =E.sub.x                 (1)

    e.sub.224a =-E.sub.T1 -V1; e.sub.224b =E.sub.T1 -V1        (2)

    e.sub.226A =-E.sub.T2 -V2; e.sub.226b =E.sub.T2 -V2        (3)

    ST=(e.sub.222a 2-e.sub.222b 2)+(e.sub.224a 2-e.sub.224b 2)+(e.sub.226a 2-e.sub.226b 2)                                           (4)

    ST=E.sub.x 2-E.sub.x 2+(E.sub.T1 -V.sub.1).sup.2 -(E.sub.T1 -V.sub.1).sup.2 +(E.sub.T2 -V.sub.2).sup.2)-(E.sub.T2 -V.sub.2).sup.2     (5)

    ST=+4E.sub.T1 V.sub.1 +4E.sub.T2 V.sub.2                   (6)

A tuning fork gyroscope according to the present invention utilizingelectrostatic electronics for both drive and sensing is illustrated inFIG. 12 by a pivoting closed-end tuning fork gyroscope 252. Thegyroscope includes plate 254 comprised of two halves, each halfelectrically isolated by means of gaps and lap joints 256,258. In thisembodiment, inverted and non-inverted driving voltages are applied tothe flexures. The inverted and non-inverted driving voltages cause thetuning fork structure, which looks capacitive as represented bycapacitor C_(f), FIG. 13, to vibrate laterally. As in the previousembodiment describing electromagnetic electronics, an excitation voltageE_(x) as well as a bias voltage B, typically 5 volts, are provided. Thevoltages (V) across capacitors C3a-C5b are represented by equations 7-12below.

    V.sub.C3a =(-E.sub.f -E.sub.x -B)-(-V.sub.1)               (7)

    V.sub.C5a =(-E.sub.f -E.sub.x -B)-(-V.sub.2)               (8)

    V.sub.C3b =(E.sub.f +E.sub.x +B)-(-V.sub.1)                (9)

    V.sub.C5b =(E.sub.f +E.sub.x +B)-(-V.sub.2)                (10)

    V.sub.C4a =-(E.sub.x +E.sub.f +B)                          (11)

    V.sub.C4b =(E.sub.x +E.sub.f +B)                           (12)

The torque (ST) measured by the gyroscope is illustrated by equations13-15.

    ST.sub.1 =[-(E.sub.f +E.sub.x +B)+V.sub.1)].sup.2 -[(E.sub.f +E.sub.x +B)+V.sub.1 ].sup.2 =-4V.sub.1 [E.sub.f +E.sub.x +B]      (13)

    ST.sub.2 =[-(E.sub.f +E.sub.x +B)+V.sub.2)].sup.2 -[(E.sub.f +E.sub.x +B)+V.sub.1 ].sup.2 =-4V.sub.2 [E.sub.f +E.sub.x +B]      (14)

    ST=ST.sub.1 +ST.sub.2 =-4[V.sub.1 B+V.sub.2 B+E.sub.f (V.sub.1 +V.sub.2)+E.sub.x (V.sub.1 +V.sub.2)]                     (15)

An additional embodiment of the tuning fork gyroscope of the presentinvention is illustrated as a closed end, non-pivoting micromechanicaltuning fork gyroscope 260, FIG. 14. In contrast to a pivoting plategyroscope previously described in conjunction with FIGS. 10 and 12wherein angular rotation about the input axis produces a tilt orrotation in the plate, the tuning forks in non-pivoting gyroscopesexperience an upward force on one fork while the other fork experiencesa downward force. Thus, the non-pivoting tuning fork gyroscope 260includes two parallel vibratable structures or forks 262 and 264 whichare driven by an AC voltage source (E_(f)) 266. The voltage causes thetuning forks to vibrate laterally, in the plane of the structure.Angular rotation about the gyroscope's input axis indicated generally byarrow 268 causes the tuning forks to vibrate in a vertical direction,perpendicular to the lateral direction.

Angular rotation or readout is performed electrostatically by one ormore electrodes above or below the tines such as electrodes 270 and 272and utilizing the illustrated circuit which is similar to the circuitspreviously described.

As previously discussed, the tuning fork gyroscope and electroniccircuits of the present invention can be operated in two modes namely,closed-loop or open-loop. The circuit described in connection with FIGS.10 and 11 illustrates an open-loop mode configuration. A closed-loopcircuit is illustrated by box 274, FIG. 15 wherein there are four inputsnamely E_(T1) (W_(v) 0 or vibration frequency angle 0), E_(T2) (W_(v) 90or vibration frequency, angle 90), V₁ and V₂. Output 276 from thegyroscope is an AC signal, carrier modulated. This signal is thendemodulated and filtered by demodulator/filter 278, against theexcitation signal E_(x) which strips out the carrier. This produces anAC signal 280 at the vibration frequency which has an amplitudeproportional to the angular rate of the gyroscope. The signal is then ACamplified by amplifier 282 and again demodulated and filtered againstthe vibration frequency at angle 0 by demodulator/filter 284, andagainst the vibration frequency at angle 90 by demodulator/filter 286.The output signals from demodulator/filters 284,286 are then conditionedwhich provide quadrature and gyroscope signals 288 and 290 respectivelywhich are fed back to the input of the gyroscope so that the plate ismaintained at AC null in both phases. The gyroscope signal output 290 isthe desired gyroscope output, whose voltage is proportional to theangular input rate.

The circuit described in conjunction with FIG. 15 applies when thevibration frequency of the tuning forks or vibratable structures is notequal to the natural frequency of the pivoting gyroscope plate. Theelectronics may also be suitably modified to allow operation when thevibration frequency of the tuning forks is equal to the naturalfrequency of the gyroscope plate. Such a modification is well known tothose skilled in the art.

An additional embodiment of the micromechanical tuning fork gyroscope ofthe present invention is shown in FIG. 16. The selection of such anembodiment is driven by the desire to maximize the sensitivity of thedevice. To achieve this goal, the desire is to maximize the gyroscopictorque on the pivoting tilt plate 354.

As shown in equation 16 below, if all the inertia of the tuning forkgyroscope according to the present invention were concentrated in thetuning forks or vibrating tines themselves, then the gyroscopic torqueproduced when an input rate is applied is shown in equation 16 where I₁is the change in the moment of inertia about the input axis (I_(x)) asthe tuning fork oscillates.

    Gyroscopic torque=I.sub.1 w.sub.t W.sub.x cos w.sub.t t    (16)

I_(x)

where

I₁ =DI_(x) =2Mr Dr;

w_(t) is the tine frequency;

t is time;

W_(x) is the input rate;

M is the mass of the tuning forks; and

r is the deflection of the tine along the Y axis.

Since r is the deflection of the tuning fork tine, the tines should belocated as far apart as possible to maximize the gyroscopic torque for agiven deflection of the tines. Stated differently, the force on thetines from the gyroscopic action is fixed for a given tine deflectionand therefore, by placing the tines as far as possible from the axis ofrotation, the torque on the tilt plate is maximized. The largergyroscopic torque in effect increases the angular momentum which resultsin more volts per unit rate input, raising the signal level, reducingthe gyroscope's sensitivity to Brownian noise, thus making design of thegyroscope and the electronics easier.

As shown in FIG. 16, the vibrating tines 350,352 represented, for thesake of simplicity, by simple bars F and G, are located on the extremeedge of tilt or pivoting plate 354. The tines are electrically isolatedfrom the rest of the tilt plate by dielectric or P-N isolation regions356a-356d. The tines are connected by a wire 358 which is dielecricallyisolated from the rest of the structure and connected to input flexurepivot 360 (E) also dielectrically isolated from the remainder of thestructure. Wire 358 places a common input voltage E_(t) on eachvibrating tine 350,352.

Output flexure pivot 362, also dielectrically isolated, in turn isconnected to the input node (N) of operational amplifier 364 as will befurther explained below. Thus, the potential of tilt plate 354 is drivento that of the operational amplifier input node (N) which in turn isdriven to virtual ground by the feedback configuration of operationalamplifier 364.

Upon application of a sinusoidal voltage (E_(t)) on input flexure 360(E) an electrostatic force is generated between the tines 350,352 andthe adjacent tilt plate 354 due to the potential difference between thetines and the tilt plate. This voltage potential difference will causethe tines to vibrate at an amplitude which is dependent upon the appliedvoltage, the frequency of the applied voltage, and the mechanicalcharacteristics of the tines.

To minimize the required tine drive voltage, it is most desirable todrive the tines at their resonant frequency. In order to accomplishthis, the displacement distance moved by the tines must be measured.This may be accomplished by the circuit shown in accompanying FIG. 17including the loop formed by E, N, I and E_(t).

In this circuit, an AC voltage E_(xt) at a typical frequency of greaterthan 100 KHz, which is much higher than the resonant frequency of thetines, typically 3 to 10 KHz is applied as shown. This AC signal is usedas a carrier and is modulated by the tine vibration.

The capacitance labeled C_(t) is the capacitance of the tine gap. Achange in capacitance C_(t) is a function of the displacement of thetines. Thus, the output voltage of operational amplifier 364 isproportional to the capacitance C_(t) of the tines, and the change inthe output voltage of the operational amplifier is similarlyproportional to the change in capacitance C_(t) of the tines. Since theoutput of the operational amplifier is a function of the tinecapacitance, the output can be used to provide an indication of tineposition. This may be done by demodulating the output 366 of theoperational amplifier with respect to the tine excitation frequencyw_(xt). The output (I) 368 of the demodulator is appropriately modifiedby feedback network 370 to provide the tine voltage E_(t). This willcause the tines to oscillate at their resonant frequency w_(t). A fixedbias voltage B_(t) of typically 1-10 volts, is also summed in with thetine voltage E_(t) and the tine vibratory frequency E_(xt) to linearizethe force characteristics.

As a result of an input rate along the gyroscope's input axis, the tiltplate 358 will oscillate about the input axis indicated generally byarrow 372, with the angle of vibration proportional to the input rate.This angle or amount of vibration of the tilt plate must then bemeasured. This may be accomplished by including sense electrodes 374,376which may run over (bridge) or under (buried) tilt plate 358. The senseelectrodes are represented as nodes A and B in the circuit diagram ofFIG. 17.

An AC excitation voltage E_(xs) of frequency w_(xs), typically equal toor greater than 100 KHz, and much higher than the vibration frequencybut different than the tine excitation frequency w_(xt) is applied. TheAC excitation voltage E_(xs) is applied directly to one sense electrodeand inverted with respect to the other electrode.

When the tilt plate is level, that is, no input angular rate is present,the capacitances between the two sense electrodes and the tilt plate areequal and therefore, there is no output from operational amplifier 364.When the plate tilts as a result of an input angular rate, thecapacitances C_(x1) and C_(x2) change differentially and therefore, avoltage is generated by operational amplifier 364 which is proportionalto the difference in capacitances. The output of operational amplifier364 is demodulated by demodulator 378 with respect to the tineexcitation frequency w_(xs) to provide an output signal 380 that is afunction of the tilt angle of the plate and thus, a function of theinput angular rate.

Further, torquing electrodes 382,384, labeled as nodes C and D, areprovided to enable a rebalance torque to be applied to the tilt plate.As shown in FIG. 17, a rebalance voltage E_(R) and a linearizing biasB_(R) are summed and applied to the torquing electrodes.

In the embodiment described immediately above, discussions were focusedon the gyroscope in general, the tines having been represented as simplebars. In a preferred embodiment, however, such a structure would notafford as efficient of a vibratable structure as desired. The mostefficient vibratable structure must be flexible horizontally that is,within the plane of the device, while rotationally and vertically stiff.

Accordingly, the preferred embodiment of a pivoting, micromechanicaltuning fork of the present invention includes the tine structure showngenerally in FIG. 18 which includes a mass 400 attached to tilt plate402 by means of four tines 404a-404d which act like "springs". Acapacitative gap indicated generally by arrow 406 is formed between themass and the adjacent plate 402. Tines 404a-404d are high aspect ratiobeams and provide no other function other than to allow the mass tovibrate parallel to axis 408. The vibrating frequency of such avibratable structure is a function of this supporting tines 404a-404dand the central mass. The mass of the supporting tines is negligible.This avoids undesirable effects that might occur due to multi-modevibration of the supporting tines or springs. Further, the verticalrigidity of the supporting tines in all directions but laterally meansthat there will be no problems with other resonants either vertically,out of plane or rotationally. This approach is in accordance with thedesired principal of isolation, which requires one function for eachstructural element. Further, the four supporting tine structures providethe most rotational stiffness which tends to minimize effects due tomass unbalance.

Although capacitive gap 406 is formed by simple parallel structures, analternative embodiment includes the interdigital gap structure shown inFIG. 19.

An additional embodiment of a pivoting, micromechanical tuning forkaccording to the present invention is illustrated in FIG. 20 which,although similar to the pivoting tuning fork gyroscope shown in FIG. 16,is adapted to utilize electromagnetic drive and electrostatic sense andrebalance torquing electronics. In this embodiment, the vibrating tinessuch as tine 410, again illustrated as a simple bar for the sake ofsimplicity, is disposed between two conductive electrodes 412,414. Thetines are driven electromagnetically by properly controlling thedirection of current flow through the tine and the adjacent conductiveregions of the tilt plate.

The direction of current flow is best illustrated by the schematicdiagram of FIG. 21. To fabricate such a structure, the tines may besufficiently doped to carry enough current. Alternatively, anelectrically isolated metallization layer may be deposited on top of thetines. Similarly, the areas of the tilt plate adjacent the vibratingtines may be similarly doped or include electrically isolatedmetallization layers. Electrostatic sense and torquing electronics,similar to those described in conjunction with FIG. 17 are contemplated.

Although specific features of the invention are shown in some drawingsand not in others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention. Additionally, although the structure of the presentmicromechanical tuning fork angular rate sensor has been described inconjunction with a structure a unitary silicon substrate and anisotropicetching, this is not a limitation of the present invention.

For example, in addition to the silicon bulk micromachining andanisotropic etching techniques described above, the structure andfabrication of a vibrating structure on a gimbal as an angular ratesensor of the present invention may be fabricated from various othertechniques. These include defining the vibrating and gimbal componentsby reactive ion etching of the surrounding silicon after diffusing withBoron as a P+ region to form the broad outline of the structure,followed by anisotropic etching. The device may be built up from thesurface of a material as in the case of selectively applying polysiliconover a silicon wafer. Further, bonding techniques may be utilized tocreate a layered or sandwiched structure utilizing silicon, glass andquartz.

For example, an alternate method of fabricating a micromechanical tuningfork angular rate sensor according to the present invention includes asurface micromachining process which builds up the structure forming thetuning fork angular rate sensor as opposed to building the structurefrom the top down as described above in conjunction with selectiveanisotropic etching techniques.

As shown in FIG. 22, this embodiment of the tuning fork angular ratesensor 500 is fabricated by first providing a substrate 502 which may besilicon. A thin layer 504 of silicon nitride is applied over thesubstrate 502 to protect the substrate from subsequent processing.

Next, a sacrificial layer 510 is applied over the substrate. It ispatterned to remain temporarily where gaps or undercutting are requiredbetween the tines and the substrate. The sacrificial layer may include aCVD glass layer of approximately two microns in thickness. The CVD glasssacrificial layer etches very rapidly in a buffered HF water etchsolution.

A thick layer of polysilicon 512 is then deposited over the sacrificiallayer 510 and its upper surface 514 oxidized. The surface oxide ispatterned and used as a mask to plasma etch the polysilicon layer 512leaving only the tines 516a, 516b of the tuning fork angular rate sensorand flexures (not shown) which couple the tines to the substrate. In thecase of open ended tines, one flexure is provided proximate one end ofthe tines. In the case of close ended tines, two flexures are provided,one proximate each of the two ends of the flexures. The sacrificiallayer 510 is then etched away to undercut and release the tines. Usingthis method, aspect ratios of 1 to 4 for the tines as well as otherstructures may be achieved.

Additionally, read-out electrodes may be provided which are implementedas P/N junctions under the device, as a thin layer of metallization or athin layer of polysilicon under the sacrificial layer 510.

Yet another embodiment of the present invention contemplates thefabrication of a tuning fork angular rate sensor utilizing a layered orsandwiched method. As shown in FIG. 23, the tuning fork angular ratesensor 520 includes a base electrode layer 522 over which is applied abeam layer 524 followed by a top electrode layer 526. According to thistechnique, this embodiment of the micromechanical tuning fork angularrate sensor allows beam layer 524 to be fabricated utilizing any one ofthe many etching techniques now known or which may be discovered in thefuture to completely etch through a thin wafer. Oxide or plated spacers528a-528d are provided proximate beam layer 524 to allow the tuningforks to vibrate and rotate as previously described. Electrodes 530 and532 are contained on the top and base electrode layers 526, 522respectively. The electrodes may include doped silicon regions in thecase of silicon layers or deposited metallized electrodes in the case ofglass or quartz layers. Well known wafer bonding techniques are utilizedto bond the wafers or layers together.

These, and other structures and fabrication techniques for therealization of a gimballed vibrating pair of structures are discernableby reference to FIG. 24. As shown there, first and second vibratingbands 540, 542 with or without additional masses, are fixed to or withina gimbal ring 544, which is in turn attached to a surrounding plate orplatform via flexures 546 located within a central zone 548.

In the first embodiment, the elements 540, 542, 544 and 546 are definedby Boron diffusions and the elements are released from the surroundingsemiconductor by anisotropic etching.

In a further embodiment, a substantial portion of the surface includingthe elements 540, 542, 544 and 546 are Boron doped. A reactive ion etch(RIE) then erodes deep pits to define the elements 540, 542, 544 and 546with high aspect ratios (height to width), etching then frees theundersurfaces.

In yet another embodiment, a layer including the elements 540, 542, 544and 546 is grown as polysilicon over a layer 550 of oxide on anunderlying silicon substrate 552. The elements 540, 542, 544 and 546 arethen defined by RIE etching the surrounding silicon and chemicallyremoving the oxide layer.

In a further embodiment, the elements 540, 542, 544 and 546 are exposedin an RIE etch and then doped with Boron. A subsequent etch frees thedesired structure.

In a fifth embodiment, the elements 540, 542, 544, 546 are vapordeposited around and on patterns formed in photo resistant and/or oxidewhich are then chemically removed to leave the gimballed structure.

Electrodes can be formed using the isolation techniques and bridge orburied electrode designs shown above.

Modifications and substitutions by one of ordinary skill in the art areconsidered to be within the scope of the present invention which is notto be limited except by the claims which follow.

We claim:
 1. A monolithic, micromechanical tuning fork gyroscope, for detecting angular rotation about at least a first rotation sensitive axis, comprising:a silicon substrate over which is suspended a silicon structure; said silicon structure disposed within a first plane and including at least first and second closed-ended elongate vibratable structures having, in line, a central mass and resilient attachments to said silicon structure, said first and second closed-ended vibratable structures disposed generally adjacent and parallel to one another; drive means, for energizing said first and second closed-ended vibratable structures to vibrate laterally along an axis normal to said rotation sensitive axis and within said first plane, said lateral vibration of said first and second closed-ended vibratable structures effecting simultaneous vertical movement parallel to a second plane and normal to said first plane of at least a portion of said silicon structure upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; and means for sensing said simultaneous vertical movement of said at least a portion of said silicon structure, and for providing a voltage output signal proportional to said sensed vertical movement, said voltage output signal providing an indication of angular rotation detected by said gyroscope.
 2. The gyroscope of claim 1 wherein said first closed-ended vibratable structure includes a first end coupled to a first segment of said silicon structure, and a second end coupled to a second segment of said silicon structure, said second segment located generally diametrically opposed from said first segment; andsaid second closed-ended vibratable structure is disposed parallel to said first closed-ended vibratable structure, and includes a first end coupled to said first segment of said silicon structure, and a second end coupled to said second segment of said silicon structure.
 3. The gyroscope of claim 2 wherein each of said first and second closed-ended vibratable structures include a mass disposed generally about a longitudinal central point of each of said first and second closed-ended vibratable structures.
 4. The gyroscope of claim 3 wherein each of said mass include a silicon mass on which is disposed a counter weight.
 5. The gyroscope of claim 4 wherein said counter weight is selected from a high density group consisting of gold, tungsten and lead.
 6. The gyroscope of claim 4 wherein each of said silicon masses is integral with said first and second vibratable structures respectively.
 7. The gyroscope of claim 6 wherein said mass on each of said first and second closed-ended vibratable structures includes a center of gravity within said first plane.
 8. A monolithic, micromechanical tuning fork gyroscope, for detecting angular rotation about at least a first rotation sensitive axis, comprising:a silicon substrate over which is suspended a silicon structure; said silicon structure disposed within a first plane and including at least first and second closed-ended elongate vibratable structures having, in line, a central mass and resilient attachments to said silicon structure, said first and second closed-ended vibratable structures disposed generally adjacent and parallel to one another; drive means, for energizing said first and second closed-ended vibratable structures to vibrate laterally along an axis normal to said rotation sensitive axis and within said first plane, said lateral vibration of said first and second closed-ended vibratable structures effecting simultaneous vertical movement parallel to a second plane and normal to said first plane of at least a portion of said silicon structure upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; means for sensing said simultaneous vertical movement of said at least a portion of said silicon structure, and for providing a voltage output signal proportional to said sensed vertical movement, said voltage output signal providing an indication of angular rotation detected by said gyroscope; first and second flexible elements coupling said silicon structure to said substrate, each of said flexible elements integral with said substrate and said silicon structure, and disposed generally co-linear and co-planar with said first rotation sensitive axis, for allowing said silicon structure to rotate about said first rotation sensitive axis; said first flexible element including a first end coupled to a first region of said silicon substrate and a second end coupled to a first side of said silicon structure; said second flexible element including a first end coupled to a second region of said silicon substrate diametrically opposed from said first region of said silicon substrate and said first region of said silicon substrate and said first flexible element, and a second end coupled to a second side of said silicon structure; said silicon structure including first and second segments, said first segment structurally coupled and electrically isolated from said second segment; said first closed-ended vibratable structure including a first end coupled to a first region of said first segment, and a second end coupled to a second region of said first segment, said second region located generally diametrically opposed from said first region; said second closed-ended vibratable structure including a first end coupled to a first region of said second segment, and a second end coupled to a second region of said second segment, said second region located generally diametrically opposed from said first region; wherein said drive means is operative for energizing said first and second closed-ended vibratable structures to vibrate laterally, co-planar with and along an axis normal to said first rotation sensitive axis, and vibrating of said first and second vibratable structures effecting rotational movement of said silicon structure about said rotation sensitive axis upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; and wherein said means for sensing senses rotation of said silicon structure, for providing a voltage output signal proportional to the rotational movement of said silicon structure, said voltage output signal providing an indication of angular rotation detected by said gyroscope.
 9. The gyroscope of claim 8 further including means for providing stress relief of tensile forces between said silicon structure and said silicon substrate.
 10. The gyroscope of claim 9 wherein said means for providing stress relief includes at least one tension relief beam formed by an opening having a predetermined length and width disposed in said silicon structure, said at least one tension relief beam coupled to the second end of at least one of said first and second flexible elements.
 11. The gyroscope of claim 10 further including a second opening having a predetermined length and width, and forming a second tension relief beam to which is coupled the second end of the other of said first and second flexible elements.
 12. The gyroscope of claim 10 wherein said at least one tension relief beam has a height-to-width ratio greater than one.
 13. The gyroscope of claim 12 wherein said height-to-width ratio is at least two-to-one.
 14. The gyroscope of claim 8 wherein said first and second segments are structurally coupled and electrically isolated by means of a lap joint.
 15. The gyroscope of claim 8 wherein said first and second segment are structurally coupled and electrically isolated by means of a dielectric isolation region.
 16. The gyroscope of claim 8 further including rebalance means, responsive to said means for sensing, for counteracting and zeroing the rotational movement of said silicon structure about said rotation sensitive axis upon the occurrence and sensing of angular rotation of said gyroscope about said rotation sensitive axis.
 17. The gyroscope of claim 1 wherein said drive means applies a sinusoidal voltage to each of said first and second closed-ended vibratable structures.
 18. The gyroscope of claim 17 wherein said sinusoidal voltage applied at a given instant to said first closed-ended vibratable structure is of opposite polarity to the sinusoidal voltage applied at said given instant to said second closed-ended vibratable structure.
 19. The gyroscope of claim 1 further including at least one sense electrode sensing movement of said at least a portion of said silicon structure.
 20. The gyroscope of claim 19 wherein said at least one sense electrode includes at least one bridge electrode coupled to said silicon substrate.
 21. The gyroscope of claim 20 wherein said at least one bridge electrode is electrically isolated from said silicon substrate.
 22. The gyroscope of claim 19 wherein said at least one sense electrode includes at least one buried electrode.
 23. The gyroscope of claim 1 wherein said means for sensing rotation includes electrostatic sense means, for electrostatically sensing rotation of said silicon structure.
 24. The gyroscope of claim 23 wherein said electrostatic sense means includes means for measuring differential capacitance between said first and second segments of said silicon structure and at least one sense electrode.
 25. The gyroscope of claim 1 further comprising a second, non-etched silicon structure, said second silicon structure including third and fourth closed-ended vibratable structures, for forming at least a second micromechanical tuning fork gyroscope disposed co-planar with a first micromechanical tuning fork gyroscope, and oriented co-planar with and orthogonal to said first tuning fork gyroscope, for detecting angular rotation about a second rotation sensitive axis co-planar with and generally orthogonal to said first rotation sensitive axis.
 26. The gyroscope of claim 25 further including a third micromechanical tuning fork gyroscope disposed in a plane orthogonal to said first plane, for detecting angular rotation about a third rotation sensitive axis orthogonal to said first and second rotation sensitive axes.
 27. The gyroscope of claim 25 further includinga third gyroscope having a third rotation sensitive axis normal to the plane of the first and second tuning fork gyroscopes and orthogonal to the rotation sensitive axes of the first and second tuning fork gyroscopes; and first, second and third accelerometers, for detecting acceleration along said first, second and third rotation sensitive axes respectively, and forming along with said first, second and third gyroscopes a three axis instrument measurement unit on one silicon substrate.
 28. The gyroscope of claim 1 wherein said drive means includes electrostatic drive means.
 29. The gyroscope of claim 1 wherein said drive means includes at least first and second electrodes, for applying a drive voltage to said first and second closed-ended vibratable structures.
 30. The gyroscope of claim 1 wherein each of said first and second closed-ended vibratable structures includes:first and second support beams, each of said first and second support beams including a first end coupled to a first region of said silicon substrate, and a second end coupled to a first region of said mass; and third and fourth support beams, each of said third and fourth support beams including a first end coupled to a second region of said silicon substrate, and a second end coupled to a second region of said mass, said second region of said mass and said second region of said silicon substrate located generally diametrically opposed from said first region of said mass and said first region of said silicon substrate respectively.
 31. The gyroscope of claim 30 wherein each of said support beams include first and second portions, said first portion proximate said first end and coupled to said silicon substrate; andsaid second portion disposed proximate said second end and coupled to said mass.
 32. A pivoting, monolithic, micromechanical tuning fork gyroscope, for detecting angular rotation about at least a first rotation sensitive axis, comprising:a silicon substrate over which is suspended a silicon structure; first and second flexible elements coupling said silicon structure to said substrate, each of said flexible elements integral with said substrate and said silicon structure, and disposed generally co-linear and co-planar with said rotation sensitive axis, for allowing said silicon structure to rotate about said rotation sensitive axis; said first flexible element including a first end coupled to a first region of said silicon substrate and a second end coupled to a first side of said silicon structure; said second flexible element including a first end coupled to a second region of said silicon substrate diametrically opposed from said first region of said silicon substrate, and a second end coupled to a second side of said silicon structure, diametrically opposed from said first side of said silicon structure; said silicon structure including first and second closed-ended vibratable structures, said first and second closed-ended vibratable structures disposed generally parallel to one another, each of said first and second closed-ended vibratable structures including a central mass and, in line, resilient attachments to said silicon substrate integral with an associated vibratable structure; said first closed-ended vibratable structure including a first end coupled to a first region of said silicon structure, and a second end coupled to a second region of said silicon structure, said second region located generally diametrically opposed from said first region; said second closed-ended vibratable structure including a first end coupled to a third region of said silicon structure, and a second end coupled to a fourth region of said silicon structure, said fourth region located generally diametrically opposed from said third region; drive means, operative for energizing said first and second closed-ended vibratable structures to vibrate along an axis co-planar with and normal to said rotation sensitive axis, vibration of said first and second closed-ended vibratable structures effecting rotational movement of said silicon structure about said rotation sensitive axis upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; and means for sensing rotation of said silicon structure, and for providing a voltage output signal proportional to the rotational movement of said silicon structure, said voltage output signal providing an indication of angular rotation detected by said gyroscope.
 33. The gyroscope of claim 32 further including rebalance means, responsive to said means for sensing, for counteracting and zeroing the rotational movement of said silicon structure about said rotation sensitive axis upon the occurrence and sensing of angular rotational of said gyroscope about said rotation sensitive axis.
 34. A pivoting, monolithic, micromechanical tuning fork gyroscope, for detecting angular rotation about at least a first rotation sensitive axis, comprising:a silicon substrate over which is suspended a silicon structure; first and second flexible elements coupling said silicon structure to said substrate, each of said flexible elements integral with said substrate and said silicon structure, and disposed generally co-linear with said rotation sensitive axis, for allowing said silicon structure to rotate about said rotation sensitive axis; said first flexible element including a first end coupled to a first region of said silicon substrate and a second end coupled to a first side of said silicon structure; said second flexible element including a first end coupled to a second region of said silicon substrate diametrically opposed from said first region of said silicon substrate and said first flexible element, and a second end coupled to a second side of said silicon substrate; said silicon structure including first and second segments, said first segment structurally coupled and electrically isolated from said second segment; said silicon structure further including first and second closed-ended vibratable structures, said first and second closed-ended vibratable structures disposed generally parallel to one another, each of said first and second closed-ended vibratable structures including a central mass and, in line, resilient attachments to said silicon substrate integral with an associated vibratable structure; said first closed-ended vibratable structure including a first end coupled to a first region of said first segment, and a second end coupled to a second region of said first segment, said second region located generally diametrically opposed from said first region; said second closed-ended vibratable structure including a first end coupled to a first region of said second segment, and a second end coupled to a second region of said second segment, said second region located generally diametrically opposed from said first region; drive means, operative for energizing said first and second closed-ended vibratable structures to vibrate along an axis co-planar with and normal to said rotation sensitive axis, vibration of said first and second closed-ended vibratable structures effecting rotational movement of said silicon structure about said rotation sensitive axis upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; and means for sensing rotation of said silicon structure, and for providing a voltage output signal proportional to the rotational movement of said silicon structure, for providing an indication of angular rotation detected by said gyroscope.
 35. The gyroscope of claim 33 further including rebalance means, responsive to said means for sensing, for counteracting and zeroing the rotational movement of said silicon structure about said rotation sensitive axis upon the occurrence and sensing of angular rotational of said gyroscope about said rotation sensitive axis.
 36. A monolithic, micromechanical tuning fork gyroscope, for detecting angular rotation about at least a first rotation sensitive axis, comprising:a silicon substrate over which is suspended a silicon structure; said silicon structure disposed within a first plane and including at least first and second closed-ended vibratable structures, said first and second closed-ended vibratable structures disposed generally adjacent and parallel to one another; first and second flexible elements coupling said silicon structure to said substrate, each of said flexible elements integral with said substrate and said silicon structure, and disposed generally co-linear and co-planar with said first rotation sensitive axis, for allowing said silicon structure to rotate about said first rotation sensitive axis; said first flexible element including a first end coupled to a first region of said silicon substrate and a second end coupled to a first side of said silicon structure; said second flexible element including a first end coupled to a second region of said silicon substrate diametrically opposed from said first region of said silicon substrate and said first flexible element, and a second end coupled to a second side of said silicon structure; said silicon structure including first and second segments, said first segment structurally coupled and electrically isolated from said second segment by means of a dielectric isolation region; said first closed-ended vibratable structure including a first resilient end coupled to a first region of said first segment, and a second resilient end coupled to a second region of said first segment, said second region located generally diametrically opposed from said first region; said second closed-ended vibratable structure including a first resilient end coupled to a first region of said second segment, and a second resilient end coupled to a second region of said second segment, said second region located generally diametrically opposed from said first region; drive means, for energizing said first and second closed-ended vibratable structures to vibrate laterally along an axis normal to said rotation sensitive axis and within said first plane, said lateral vibration of said first and second closed-ended vibratable structures effecting simultaneous vertical movement parallel to a second plane and normal to said first plane of at least a portion of said silicon structure upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; wherein said drive means is operative for energizing said first and second closed-ended vibratable structures to vibrate laterally, co-planar with and along an axis normal to said first rotation sensitive axis, and vibrating of said first and second vibratable structures effecting rotational movement of said silicon structure about said rotation sensitive axis upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; means for sensing said simultaneous vertical movement of said at least a portion of said silicon structure, and for providing a voltage output signal proportional to said sensed vertical movement, said voltage output signal providing an indication of angular rotation detected by said gyroscope; and wherein said means for sensing senses rotation of said silicon structure, for providing a voltage output signal proportional to the rotational movement of said silicon structure, said voltage output signal providing an indication of angular rotation detected by said gyroscope.
 37. A monolithic, micromechanical tuning fork gyroscope, for detecting angular rotation about at least a first rotation sensitive axis, comprising:a silicon substrate over which is suspended a silicon structure; said silicon structure disposed within a first plane and including at least first and second closed-ended elongate vibratable structures having, in line, a central mass and resilient attachments to said silicon structure, said first and second closed-ended vibratable structures disposed generally adjacent and parallel to one another; drive means, for energizing said first and second closed-ended vibratable structures to vibrate laterally along an axis normal to said rotation sensitive axis and within said first plane, said lateral vibration of said first and second closed-ended vibratable structures effecting simultaneous vertical movement parallel to a second plane and normal to said first plane of at least a portion of said silicon structure upon the occurrence of angular rotation of said gyroscope about said first rotation sensitive axis; and means for sensing said simultaneous vertical movement of said at least a portion of said silicon structure, and for providing a voltage output signal proportional to said sensed vertical movement, said voltage output signal providing an indication of angular rotation detected by said gyroscope, wherein each of said first and second closed-ended vibratable structures includes: first and second support beams, each of said first and second support beams including a first end coupled to a first region of said silicon substrate, and a second end coupled to a first region of said mass; and third and fourth support beams, each of said third and fourth support beams including a first end coupled to a second region of said silicon substrate, and a second end coupled to a second region of said mass, said second region of said mass and said second region of said silicon substrate located generally diametrically opposed from said first region of said mass and said first region of said silicon substrate respectively, wherein each of said support beams include first and second portions, said first portion proximate said first end and coupled to said silicon substrate; and said second portion disposed proximate said second end and coupled to said mass, wherein each of said second portion of each of said support beams includes a height-to-width ratio which is greater than the height-to-width ratio of each of said first portions of each of said support beams.
 38. The gyroscope of claim 37 wherein the height-to-width ratio of each of said second portion of each of said support beams is selected from the range of 0.1 to
 1. 39. The gyroscope of claim 37 wherein the height-to-width ratio of each of said first portion of each of said support beams is selected from the range of 1-10. 